Best way to control speed of 3V dc motor

Discussion in 'The Projects Forum' started by David St. Onge, Jan 2, 2015.

  1. David St. Onge

    Thread Starter New Member

    Jan 2, 2015
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    Hello,

    Newbie here. Need opinions on the best way to control the speed of a 3V dc motor. I have tried a couple of pots but the control is to condensed...in other words all the control is happening within a quarter turn. The range of RPMs I am looking for is 15-40, so pretty slow. The motor I have is Radio Shack's # 2730223

    This unit looks to do exactly what I want but it looks like a servo in the beginning of the video. Should I go that route? Or do I keep the motor I have and use gears to knock down the RPMs? Pretty confused.

    In the end, I'd like this to be battery powered and fit in a small project box.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Your pot is too high in resistance. You need one that is likely no more than 100Ω to 500Ω full resistance.
     
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  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The motor is around 2700rpm/volt so if looking for 15-40 rpm with that motor you are probably going to need another method or a different motor or even gearing?
    The video appears to be using a RC servo, but only two leads instead of the customary 3 for RC PWM but usual for the linear actuator?

    Max.
     
    Last edited: Jan 2, 2015
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  4. djsfantasi

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    Apr 11, 2010
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    Max - I don't think that's a servo; I think it's a geared motor. The video identifies it as a micro gear motor (at the beginning, they list the part numbers)

    Detail about the device is here
     
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  5. MaxHeadRoom

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    Jul 18, 2013
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    Probably identical to a RC servo without the feedback etc maybe?
    [​IMG]

    Max.
     
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  6. David St. Onge

    Thread Starter New Member

    Jan 2, 2015
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    I bought a servo from Radio Shack but didnt know how to mod it for full rotation. I took it back but have found a few video with mods on how to do this. Should I go this route?
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Look at your pot. If it has the resistance values and a letter "A", then you have a log (audio) pot and the response will be compressed. Try one with a "B" for a linear response. In any case, you will have a fairly jittery response at those RPMs with a simple resistive control and almost no 'power' (torque).

    You can try a PWM motor control for slower with better torque and speed control at low rpms.
     
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  8. Alec_t

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    Sep 17, 2013
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    If you're confident you can do the mod then that would provide a simple way of getting a geared motor relatively cheaply. However, standard servos are pretty nippy, so you will probably still need some way of reducing its speed. A suitable pot can reduce speed, but torque will also be reduced. PWM can reduce speed but still give good torque.
     
  9. #12

    Expert

    Nov 30, 2010
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    I guess I'm repeating what other people said, but...trying to modulate a very cheap, 8000 RPM motor down to 15 RPM is like trying to make a Swiss watch powered by a mouse on meth. You really need to start with something that has a chance of working.
     
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  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A RC servo has a single turn pot on it for feedback so you would need to take it apart and modify for direct DC to the motor instead of the internal single turn PWM control.
    The motor controller in your link appears to be a PWM version, there are many on ebay for around $4.00.
    Max.
     
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  11. ronv

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  12. David St. Onge

    Thread Starter New Member

    Jan 2, 2015
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    I ended up buying a RioRand mini 12V DC 60rpm High Torque Gear Box Electric motor and a 12V-40V 10A PWM DC Motor Speed controller. These two combined do exactly what I need. However the PWM prevents reverse voltage and I want the motor to run in reverse at times as well. So I added a DPDT switch after the PWM and before the motor and this works perfect. My question is, will reversing the voltage hurt the motor? Short test indicate that it seems to work fine, but not sure what might happen with longer run times.
     
  13. GopherT

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    Nov 23, 2012
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    Usually not. The only issue that I ever recalled on a DC motor is non- symmetric brushes on the motor caused unusual wear when in reverse. If you are only doing it occasionally on one of these motors (rare), even then it will not be an issue. You should be fine.
     
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  14. David St. Onge

    Thread Starter New Member

    Jan 2, 2015
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    Great. Thanks!
     
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